1063-7850/05/3104- $26.00 © 2005 Pleiades Publishing, Inc. 0339 Technical Physics Letters, Vol. 31, No. 4, 2005, pp. 339–340. Translated from Pis’ma v Zhurnal Tekhnicheskoœ Fiziki, Vol. 31, No. 8, 2005, pp. 52–56. Original Russian Text Copyright © 2005 by Simakov, Yakusheva, Grebennikov, Kisin. The phenomenon of adsorption-induced variation in the resistance of thin films of metal oxides is widely used in gas sensors [1, 2]. The miniaturization of such gas sensors leads to an increase in the field strength between electrodes, which stimulates the migration of adsorbed gas species over the surface of the active layer and influences the performance characteristics of the gas sensor [3–5]. This Letter presents the results of investigations of the current–voltage characteristics (I–U curves) of the gas sensor structures based on thin films of tin dioxide (SnO 2 ) exposed to gaseous media of various compositions. The sample structures were formed on polycrystal- line sapphire (Polycor) substrates with platinum elec- trodes preliminarily deposited via a mask onto the front side. The interelectrode gaps were 50 μm wide. Four heating elements and two thermoresistors were depos- ited in a similar technological cycle on the rear side of the substrate [6]. A tin dioxide film was deposited above the electrodes by RF magnetron sputtering of an SnO 2 /CuO target in an Ar–O 2 atmosphere [7]. The resistance of the film decreased in the presence of reducing gases in the atmosphere [8]. The oxide film thickness was ~1 μm, as measured by an SE-400 15/42 ellipsometer (Sentech Instruments GmbH, Germany). The heaters and thermoresistors were calibrated with the aid of an IR camera of the TH 3100MR type (NEC Instruments Ltd., Japan) and allowed the film temperature to be maintained at 300°C over the entire area with a lateral inhomogeneity not exceeding 5%. The I–U curves were measured using a Keithley Model 2001 digital multimeter (Keithley Instruments Inc., USA) and a Grundig Model PN-300 power supply unit (Grundig Professional Electronic GmbH, Ger- many) in a range of bias voltages from 0 to 30 V. The measurements were performed in a synthetic air (20.5% O 2 in N 2 ; Messer Griesheim GmbH, Germany) and in model gas mixtures containing CO or isopropanol (C 3 H 7 OH) vapor. The gas phase humidity was monitored by a Testo Model 615 hygrometer (Testo AG, Germany) and controlled on a 50% level in all experiments. Figure 1 shows the typical current–voltage charac- teristics of the sample structures measured in the model gas mixtures. The I–U curves were virtually linear in the range of low bias voltages (U < 5 V) and were sub- linear at higher applied voltages (U > 5 V). On a double logarithmic scale, the experimental data could be approximated by two straight segments corresponding to power functions of the type I = AU n , where I is the current, A is a constant factor, U is the bias voltage, and n is the exponent. The A and n values were different in the low- and high-bias ranges. Figure 2 presents the plots of n versus impurity con- centration in the model gas mixture. In the range of low bias voltages, the n value was about unity and it was virtually independent of the type and concentration of impurities. In the range of U > 5 V (or the field strengths above 10 3 V/cm), the n value was substantially depen- dent on the type of reducing species, while the depen- dence on their concentration was much less pronounced. This feature of the sensor characteristic can be used for recognizing the types of impurities in the gas phase. The value of the coefficient A in the range of low bias voltages is approximately inversely proportional to the sample resistance: as the impurity concentration in the gas phase increases, the sample resistance decreases (Fig. 1). This behavior is usually employed for deter- mining the impurity content in air [9]. The observed features of the I–U curves can be explained by assuming that oxygen species adsorbed on the SnO 2 film surface exhibit charging by capturing Current–Voltage Characteristics of Thin-Film Gas Sensor Structures Based on Tin Dioxide V. V. Simakov, O. V. Yakusheva, A. I. Grebennikov, and V. V. Kisin* Saratov State Technical University, Saratov, Russia “Synthesis” Company, Saratov, Russia * e-mail: kisin@sstu.ru Received November 25, 2004 Abstract—The experimental current–voltage (I–U) curves of thin-film structures based on tin dioxide (SnO 2 ) exhibit nonlinearity in the range of strong applied electric fields. The results of I–U measurements are inter- preted within the framework of a model that assumes the drift of adsorbed ions over the film surface. The observed phenomenon can be used both for detecting the impurities in air and for recognizing the types of adsorbed species. © 2005 Pleiades Publishing, Inc.